Energetic materials are those that rapidly convert chemical enthalpy to thermal enthalpy. These materials are commonly known as explosives, propulsion fuels and pyrotechnics. Thermite is a well-known subgroup of pyrotechnics. It is a combination of a fuel and an oxidizer that combusts in a self-propagating reaction producing temperatures of several thousand degrees. Either alone or in combination with other high energy materials, thermites are used for various applications that include military, mining, demolition, precision cutting, explosive welding, surface treatment and hardening of materials, pulse power applications, sintering-aid, biomedical applications, microaerospace and satellite platforms. In solid form, thermite is often a first metal and the oxide of a second metal, such as aluminum and iron oxide.
Self-propagating high temperature synthesis (“SHS”) relates to the synthesis of compounds that combust in a wave of chemical reaction that propagates over the reactants, producing a layer-by-layer heat transfer. Properties such as burn rate, reaction temperature and energy release are very important. In powder-based SHS materials, solid fuel and oxidizer are ground into fine micron-sized particles and combined. In these systems, reactions depend strongly on the interfacial surface area between the fuel and the oxidizer which is affected by the size, impurity level and packing density of the constituent powders. Since the particle size predominates in determining particle surface area, use of smaller particles is desirable to increase the burn rate of the SHS and metastable intermolecular composites (“MIC”) material.
Even if smaller particle size is achieved, mere mixing of the fuel and the oxidizer is not sufficient to guarantee an increase in the interfacial surface area. Mixing of the powders results in a random particle distribution. In such a distribution, many of the fuel particles will be surrounded by other fuel particles. There will be many places where the oxidizer has little contact with fuel particles. To significantly increase the interfacial surface area, the particles must be specifically arranged so that a large number of fuel particles contact oxidizer particles and vice versa.
The propagation rate or energy release rate is increased by homogeneous distribution of the oxidizer and the fuel in the composite. This provides high interfacial area for fuel and oxidizer as well as reduced interfacial diffusional resistance. Thus on initiating a thermite reaction, the combustion wavefront assumes maximum hot spot density resulting in a high rate of energy release. In other words, such materials would show a higher burn rate or flame propagation rates. To have homogeneous distribution of the oxidizer and fuel, a self-assembly process can be very useful. Although a similar process has been demonstrated in several different research areas, preparation of ordered nanoenergetic structures has not been shown. In the self-assembly process, fuel particles are arranged in an orderly manner around oxidizer or vice versa.
Although solid spherical nanoparticles of both the oxidizer and fuel can be assembled to create a nanoenergetic composite, the surface area in spherical nanoparticles is generally smaller than cylindrical shaped nanoparticles. In cylindrical oxidizer nanoparticles such as nanorods, it is possible to assemble a greater number of fuel nanoparticles than spherical oxidizer nanoparticles. Such composites result in higher energy density than spherical particle assembly and releases energy through conduction mechanism. In the case of porous oxidizer, such as a sol-gel oxidizer, convection generally improves the performance. Recent inventions by others provide a technique of mixing of fuel nanoparticles during gelation of oxidizers, but in these reports, the microstructures do not show homogenous distribution of fuel nanoparticles inside porous oxidizers.
Manufacture of ordered nanoparticles is a technique known for the preparation of catalysts. This technique allows two different types of particles to be arranged into nanoparticles in an orderly fashion.